CS 241: Systems Programming Lecture 21. Binary and Formatted I/O - - PowerPoint PPT Presentation

CS 241: Systems Programming Lecture 21. Binary and Formatted
 I/O

Fall 2019

• Prof. Stephen Checkoway

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Review from last time

// Open and close a file. FILE *fopen(char const *path, char const *mode); int fclose(FILE *stream); // Read or write a character. int fgetc(FILE *stream); int fputc(int ch, FILE *stream); // Read or write a string. char *fgets(char *str, int size, FILE *stream); int fputs(char const *str, FILE *stream); // Formatted output. int fprintf(FILE *stream, char const *format, ...);

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re-reading a file

void rewind(FILE *stream);

• resets the file back to the start of the stream
• NOTE: no return value
• zero out and check errno for problems

Actually is an alias for…

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Changing location in a file

int fseek(FILE *stream, long offset, int whence);
 int fseeko(FILE *stream, off_t offset, int whence);

• Return 0 on success, -1 on failure (errno)

Reposition location in stream

• offset is number of bytes added to position specified by

whence

• SEEK_SET - start of the file
• SEEK_CUR - current position
• SEEK_END - end of the file

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Getting location in file

long ftell(FILE *stream);


• ff_t ftello(FILE *stream);
• returns current offset on success
• returns -1 and sets errno on failure

fseeko and ftello are specified by POSIX but not C

• ff_t is an integral type representing file sizes (often 8 bytes)

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How can we get the size of a file to which we have an open
 FILE *stream?

• A. off_t size = ftello(stream);
• B. fseeko(stream, 0, SEEK_SET);

• ff_t size = ftello(stream);
• C. fseeko(stream, 0, SEEK_CUR);

• ff_t size = ftello(stream);
• D. fseeko(stream, 0, SEEK_END);

• ff_t size = ftello(stream);
• E. off_t pos = ftello(stream);


fseeko(stream, 0, SEEK_END);


• ff_t size = ftello(stream);


fseeko(stream, pos,
 SEEK_SET);

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int get_file_size(char const *path, off_t *size) { FILE *fp = fopen(path, "rb"); if (!fp) return -1; int ret = -1; if (fseeko(fp, 0, SEEK_END) == 0) { if ((*size = ftello(fp)) != -1) ret = 0; } int err = errno; fclose(fp); errno = err; return ret; }

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int get_file_size(char const *path, off_t *size) { FILE *fp = fopen(path, "rb"); if (!fp) return -1; int ret = -1; if (fseeko(fp, 0, SEEK_END) == 0) { if ((*size = ftello(fp)) != -1) ret = 0; } int err = errno; fclose(fp); errno = err; return ret; }

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fclose() might change errno

int main(int argc, char *argv[argc]) { for (int i = 1; i < argc; ++i) {

• ff_t size;

if (get_file_size(argv[i], &size) == -1) { perror(argv[i]); } else { long long s = size; // No way to print off_t. printf("%s: %llu\n", argv[i], s); } } return 0; }

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DANGER: Format String Attacks

Don't just print arbitrary users strings printf(line); If the attacker sets the value of line they can

• Cause it to reveal other program data by printing it from the program

stack (e.g., "%x%x%x")

• Can cause it to change program data by using "%n" which stores # of

chars printed so far

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Printing to a string

int snprintf(char *str, size_t size, 
 char const *format, ...);

• Writes at most size-1 bytes into str and null terminates
• Returns number of bytes that are printed (or would be printed if the

string were large enough), negative on error char message[100]; snprintf(message, sizeof message, "%s %d",
 some_string, some_int); /* or */ size_t size = snprintf(0, 0, "%s %d", str, x); char *message = malloc(size + 1); snprintf(message, size+1, "%s %d", str, x);

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Formatted input

int scanf(const char *format, ...);

• input analog to printf()
• reads input from stdin
• uses format string to determine types
• arguments must be pointers
• common error
• Stops when
• format string is done
• input mismatch
• returns # of successfully matched items
• returns EOF on EOF (not 0)

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scanf format string interpretation

White space matches 0 or more white space characters in the input Ordinary characters are matched against non-whitespace Conversion specifications: e.g., %8lx

• % to indicate start (like printf)
• * indicates not to store the value
• number for field width
• hh, h, l: size of storage character
• conversion character (see printf)
• Most conversion specifiers skip white space (all but %[…])

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Example

#include <stdio.h> int main(void) { int pairs = 0; int x, y; while (scanf(" (%d ,%d )", &x, &y) == 2) ++pairs; printf("Read in %d valid pairs.\n", pairs); return 0; } Spaces in the format match white space characters, the %d skips white space so (1,2) are ( 3 , 4 ) both valid, but (0,1(2,3(4,5 gives 3 valid pairs!

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scanf family

int fscanf(FILE *stream, const char *format, ...); int sscanf(const char *str, const char *format, ...);

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Character ranges

%s matches a sequence of non-whitespace characters %[chars] matches a range of characters, which can include whitespace char html_tag[32];
 sscanf(line, " <%31[^>]>", html_tag);

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Assume we have an integer variable x, how do I read in a decimal value? int x;

• A. scanf("&x");
• B. scanf("%d", x);
• C. scanf("%d", &x);
• D. scanf("%d", *x);
• E. scanf("%x", &x);

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Assume we have a char array word, how do I read in a text string? char word[16];

• A. scanf("&word");
• B. scanf("%15s", word);
• C. scanf("%16s", word);
• D. scanf("%s", &word);
• E. scanf("%s", *word);

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Useful input technique

fgets() / sscanf() pairing

• Read a line using fgets()
• Parses data using sscanf() from line

Always does bounds checking

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Binary data

size_t fread(void *ptr, size_t size, size_t nitems,
 FILE *stream);
 size_t fwrite(void const *ptr, size_t size,
 size_t nitems, FILE *stream);

• Read/write nitems number of size sized objects
• Returns the number of objects read/written which will be less than

nitems for EOF or an error

• Must use feof() or ferror() to determine which occurred

int x = 42;
 float y[8];
 size_t num = fread(y, sizeof(float), 8, stream);
 num = fwrite(&x, sizeof(int), 1, stream);

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In-class exercise

https://checkoway.net/teaching/cs241/2019-fall/exercises/Lecture-21.html Grab a laptop and a partner and try to get as much of that done as you can!

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